For the second year running, the £30,000 Orange broadband prize for fiction last night went to a young writer already acquiring prodigious literary celebrity. Two years younger than last year's winner, Zadie Smith, and with one book fewer under her belt, Nigerian-born Chimamanda Ngozi Adichie, 29, took it for her 230,000-word domestic epic of the Biafra war, Half of a Yellow Sun.

Her triumph vindicated the readers who have bought 187,000 copies since the paperback was published in January and the bookmakers William Hill, who made her odds-on favourite at 13-8.

Although she was up against this year's Man Booker prize winner, Kiran Desai's The Inheritance of Loss, and a US bestseller, Anne Tyler's Digging to America, Adichie's saga inspired a rare unanimity among the judges and the readers who voted it top on the prize website.

The judges' chair for the women-only contest, the writer and broadcaster Muriel Gray, said they were "hugely impressed ... this is a moving and important book by an incredibly exciting author".

The novel - only Adichie's second - is her homage to "the tiny debris of passionate courage", her fellow Nigerian writer Chinua Achebe's phrase for the Biafran victims of the 1967-70 Nigerian civil war. Born seven years after it ended, she lost both her grandfathers among the many thousands of civilian dead.

Her narrative follows five people - a university teacher modelled on Adichie's father, his partner, the partner's twin sister and the sister's English lover -through the deluded hopes and terrors of Biafra. Achebe has praised her as " a new writer endowed with the gift of ancient storytellers".

Adichie's father was Nigeria's first professor of statistics and her mother the country's first female registrar. Her first novel Purple Hibiscus was shortlisted for the Orange in 2004 and won the Commonwealth writers best first book award.

Rodney Troubridge, fiction specialist for the bookshop chain Waterstone's, said: "We are delighted that a novel set in Africa with such important and relevant things to say about our ability to initiate and survive the horrors of war has won."

The prize founder Kate Mosse, noting that another war story, Karen Connolly's The Lizard Cage, had won the Orange award for new writers, said: "Because everything is so terrible in the world, perhaps there is a hunger now for serious books about war explored through fiction."

The two shortlisted titles believed to have come closest to beating Half of a Yellow Sun are The Inheritance of Loss and the Chinese author Xiaolu Guo's tender romantic comedy A Concise Chinese-English Dictionary for Lovers. The other shortlisted titles were Arlington Park by Rachel Cusk, and The Observations by Jane Harris.

· This article was amended on Saturday July 7 2007. In the article above, we said that this year's Orange prize winner, Chimamanda Ngozi Adichie, had been made odds-on favourite at 13-8 by the bookmakers William Hill. The odds were wrongly stated. Odds-on means that something is likely and we intended to convey the meaning that if you had bet £13 you would have made a profit of £8. We should have said either, "odds-on favourite at 13-8 on", or "odds-on favourite at 8-13".